A convergent [3 + 2] dual cycloaddition/cycloreversion process between sydnone and multi-functional maleimides is reported as an efficient protocol for thermal crosslinking of polymers. To alleviate the commonly observed pre-crosslinking problem, the reactivity of 1,3-dipolar sydnone moieties is tuned by increasing its steric effect through a sydnone-containing side-chain polymer (SCP) that is synthesized using a Mitsunobu post-functionalization method. Thin films and amorphous solid composites derived from SCP and a passive bis-maleimide cross-linker (BMI-1) are used to conduct a model study of the crosslinking process. The results from spectroscopic and thermal analyses show that the SCP/BMI-1 composites react efficiently at a modest curing temperature of ∼100 °C and can be converted into a highly cross-linked polymer network with excellent thermal stability. The spontaneity and high conversion efficiency of thermal cross-linking for SCP/BMI-1 suggest that an efficient three-step cascade reaction with minimal side reactions can be achieved in the solid-state. This lattice hardening process was implemented in a new cross-linkable electro-optic (EO) polymer SCP/BMI-2/TMI possessing a dipolar polyene chromophore crosslinker BMI-2. Through poling and in situ cross-linking, the poled films of SCP/BMI-2/TMI exhibit both large EO coefficient (r₃₃ of 117 pm V⁻¹ at 1.31 μm) and excellent long-term alignment stability at 85 °C.